1. Field of the Invention
The present invention relates generally to an internal tube sheet sealing apparatus assembly for tubular heat exchangers. More particularly, this invention relates to tubular heat exchangers of the shell-and-tube type having removable bundles such as those with U-tubes and floating heads that are designed to operate under conditions of pressures exceeding, e.g., 1000 psi on both sides of the tube sheet and wherein the tube sheet itself is usually subject to a lower differential pressure; and that have an internal tube sheet sealing apparatus.
2. Description of the Prior Art
Shell-and-tube type heat exchangers constitute the bulk of the unfired heat transfer equipment in chemical plants, petroleum refineries, steam plants, and similar installations. Heat exchangers having removable tube bundles, such as those with U-tubes and a floating head, or having non-removable bundles with fixed tube sheets are among the most popular exchangers of the principal types of shell-and-tube heat exchangers in current usage. In practice, it is desriable to hydrostatically or pneumatically test the shell side of tubular heat exchangers, including those with removable tube bundles, so that visual inspection of the tube joints and tube sheet gaskets can be made and leaks can be readily located and serviced. Generally, leak detection is not a problem. However, when leaks have to be determined without ready access to the tube ends, it may be necessary to re-roll or re-weld all the tube-to-tube sheet joints, thereby incurring substantial risk of possible damage to the satisfactory joints. The problem of leak detection becomes especially acute when the tubular heat exchanger is intended for high pressure operations exceeding 1000 psi especially those under which the pressures exceed 2000 psi on both sides of the tube sheet and the tube sheet itself is subject only to a lower differential pressure. For pressures between 1000 psi and less than 2000 psi, bolts can be used to help constrain the hydrostatic end loads. However, for pressures exceeding 2000 psi, it is advisable to use shear members. Therefore, for high pressure operations it is especially desirable to hydrostatically or pneumatically test, at differential pressure, the shell side of such tubular exchangers upon completion of shop fabrication and/or during maintenance operations to detect leaks at the tube sheet when the high pressure end closure is not (yet) in place. However, all of the above circumstances and operating conditions have led to the adoption of many different technical solutions, many of which have only been partially successful because they have only resolved some problems and introduced others.
For example, U.S. Pat. No. 2,223,320 specifically discloses how to avoid the use of bolts to resist the hydrostatic force acting on the tube sheet by utilizing a telescopic extension, welded to the shell and secured by pins or plugs held in place by solder or other material. However, the solution disclosed by this patent does not enable the tube sheet joint and tube joints to be tested without the channel cover and its loading devices employed in place. With multiple tube side passes, moreover, the use of partition means to separate such passes within the channel, has led to the development of the pass partition box, which must also be kept sealed in order to prevent leaks between passes. In such a system, therefore, the stationary tube sheet is not the only site of leaks, because the pass partition box in such a system poses another type of leakage problem by being an additional site for leaks. U.S. Pat. No. 2,956,704 relates to such a situation. This patent discloses the use of a pipe and flanges as a sealing means to replace the partition box, and permits the tube sheet joint to be tested hydrostatically in the absence of the channel cover. However, the tube sheet joint of this patent can not be re-loaded while in service.
Therefore, there has existed for a long time an unsatisfied need in the art for an internal tube sheet sealing means, especially one that can perform the dual functions of (a) sealing a stationary tube sheet in a tubular heat exchanger having a removable tube bundle and of (b) sealing a pass partition box, both functions being carried out under pressures exceeding at least 1000 psi, or 2000 psi, on both sides of the tube sheet and with the tube sheet itself being subject to a usually lower differential pressure. There has especially been an unsatisfied need for a system of tube sheet sealing means capable of effecting such functions under such pressures through the use of a special means for loading the tube sheet which not only enables the tube sheet joint and tube joints to be tested by differential pressure hydrostatic testing without the channel cover and end closure components mounted in place, but also to enable the tube sheet joint to be retightened, as needed, through the use of means mounted externally to the exchanger.